Recovery of so2 from digester blowoff gases



April l, 1947. H. A. nu Bols RECOVERY OF SO2 FROM DIGSTER BLOW-OFF GASES Filed Oct. 5, 1940 atented Apr. '1, 1947 l RECOVERY F4 SOzFROM DIGESTER BLOWOFFf GASES Horace A. Dn1 Bois, Neenah, Wis., assignor to `,Paper Patents Company, Neenah, `Wis., a corporation of `Wisconsin .,ApplicationOctober 5,1940, Serial No.- 359,835 i My invention relates, generally, `to -sulphite pulp plants, and it has particular relation. to a method and means for recoveringfand utilizing SO2 from waste gases periodically discharged in connection therewith.

In general, a sulphite pulp plant includes a digester operated in an intermittent manner, and a sulphur burner and liquor making system which operate in a continuous manner. Depending upon the size of the plant, there may beone or a battery of several digesters operated in connection with the `sulphur burner and acid liquor system. At the start of each digester batch, or

cook, a digester is loaded with wood chipsland nlled with acid liquor containing an excess of SO2. After charging', the digester is closed and the contents are steam heated to cookingtem- ,perature After aninitial cooking period, duringwhich the digester pressure remains substantially uniform, the digester is blown down preparatory to blowing the finished sulphite pulp from the digester into a blow pit.

The main purpose of the blow-down portion of the cook or batch is to recover 'asmuch SO2 gas as possible before emptying the digester contents into the blow pit, since the cost of SO2 is a very important factor or item in the production of sulphite pulp, And, normally, if ,the SO2 is not recovered during the blow-down period it is wasted to the atmosphere as part of the blow pit ventilator gases. The recovery of SO2 is considered sufficiently important even though the blowdown period may take from one-third to one-half of the tota-l cooking, time, and in spite of the fact that in many cases the conditions established or prevalent in the digester during the blow-down period have a positive detrimental eilect upon pulp yield and the quality thereof. l

The principal object of my invention is o provide an inexpensive method and means of recovering the SO2 periodically discharged as a component of the blow pit ventilator gases. It will be seen that SO2 intermittently discharged from certain other parts of the digester system may also be similarly recovered. The invention permits a shorter blow-down period, thereby increasing a pulp plants output and permitting a better quality of pulp to be produced. If increased pulp production is not essential, my invention permits the maintenance of regular production rates at lowered cooking temperatures, thereby reducing the steam cost factor, and increasing the pulp yield per unit of Wood. And, the invention permits a greater overall SO2 recovery thereby reducing chemical cost and allowing a corresponding reduction `in pulpcost.

A very important Vfeature `of my invention is 'that it involves inexpensive equipment Vand material of simple construction whichmay be conveniently added to-a conventional sulphite plant installation, The additional equipment required in connectionl with my invention is dependable :and operates in a continuous manner so vthat its adoption does not increase operating costs.

`It will be understood thatalthough, conceiv-` ably, certain prior` artstripping equipment and methods are available which could after a fashion handle these high intermittent discharges of l5y SO2, suchV equipment and methods as were known would require high initial `iirst cost and entail excessive operating costs.

l According to my invention,` the water vapor is first condensed from the blow pit ventilator gases, or other periodically discharged SOzenriched gases in connection with the digester system. This water vapor or ilash steam condensation is preferably carried out in a packed tower with a. water spray or shower at the top thereof, the condensing water being automatically supplied at such a rate that the waste Water formed is at a temperature slightly under `its boiling point or 212 F. In this way substantially all of the water vapor or steam is condensed from the SO2 enriched gases but only very little SO2 will be dissolved and carried away in the 'hot waste water.

The condensation step mayalso be carried out by means of sprays provided in the blow pit I2. In this event the condensing water supply would be controlled automatically by the temperature of the gases leaving the blow pit.

After condensation of the steam, the SO2 is 'next absorbed from the SO2 enriched gases to forman aqueous SO2 solution, The absorption of the SO2 is also preferably carried out by a waterlshower in a packed absorbing tower, the water being supplied at such a temperature and in such an amount that substantially all of the SO2 is absorbed.

The aqueous SO2 solution is then collected and l drained into a reservoir from which it may be drawn and `pumped into the continuously operating cooling system for the hot burner gases, forming a part ofthe plantsacid liquorl making system. Here the SO2 is stripped from the aqueous SO2 solution by the hot burner gases. The

reservoir` for the aqueous SO2 solution has a storage capacity suliicientto accommodate the relatively high intermittent inux of the solution a pipe 24 to a supply water main 25.

3 and to also Permit it to be continuously pumped away into the burner gas cooling system at a uniform rate.

For a more complete understanding of the nature and lscope of my linvention, reference may be had to the following detailed description, taken in connection with the` accompanying drawing wherein the single ngure is a now sheet diagrammatically illustrating my invention.

Referring now to the drawing, a sulphi'te pulp plant is shown having a digestcr IIJ, of any suitable conventional construction, with-its bottom outlet connected through a discharge pipe I I into a blow pit I2. lThe discharge pipe II is provided with a valve I3 adjacent to the outlet I4 of the digester I for shutting ofi the discharge pipe except iwhen the digester contents are being discharged into the blow pit I2. A drain pipe is provided at the lower right hand side of the blow pit I2 leading into a sewer, as shown. The drain I5 is provided with a suitable valve I 6. When the digester contents are blown into the blow pit I2 through the discharge pipe II at the end of each cook or batch, the reduction in pressure allows certain gases to be given off in the blow pit I2.l These gases are generally ref erred to as blow pit ventilator gases and contain water vapor or flash steam, sulphur dioxide, and inert gases, largely nitrogen. These blow pit ventilator gases are taken oif from the top of' the blow pit I2 and conducted through a ventibacking up into the ventilator pipes I1 of other -blow pits in the system. The condenser tower 2| is packed with suitable material of a nonfouling nature.

lIn order to condense the water vapor or flash steam from the ventilator gases rising up through the condenser tower 2|, a `water spray or shower 23jis provided which isconnected. by In order to prevent absorption of SO2 in the condenser tower 2|. the condensing or spray water supply to the shower 23 should be regulated in such amount that the waste water formed when it condenses the flash steam from the ventilator gases will be at a temperature only slightly below the boiling point or 212 F. At this temperature only an insignificant amount of SO2 can be dissolved in or absorbed by the hot waste water. In order to provide for such controlled regulation of the condensingy water, a regulating valve 26 is provided in the pipe line 24 which is automatically controlled by a thermo-responsive element or thermostat 21 provided in the base of the condensing `tower 2|, at which point the waste water collects.

This condensing water control system may be of any one of several conventional throttling types. For example, the thermo-element 2`I may be of the vapor pressure type whereby changes in temperature thereof produce corresponding changes in pressure. The pressures in turn may control the pressure of an external supply of compressed air from a separate system. The compressed air may operate an air operated diaphragm motor valve which controls the flow of condensing water. As an alternative, the ther- '4 mo-element 21 may be of an electrical type, such as a resistance thermometer. In this case, electrical means in circuit relationship with the element 21 automatically restore the system 5 to electrical balance and also operate through a System of relays to open or close the condensing water control valveA by means of currents supplied to a small reversing valve motor. A bypass pipe 30 with a valve 3| is provided in the pipe line 24 around the automatically controlled valve 26.

The hot waste watercollected in the bottom of the condenser tower 2| may be drained into a waste storage tank I8 through a drain pipe the'condenser tower 2| through a vent pipe 2|I. The hot waste water may be drained from the waste storage tank|8 into the blow pit I2 through vfrom cooking liquor. It will also be seen that hot waste water may be utilized for any otherA suitable purpose in the sulphite plant. 1 The waste storage tank I8 is provided with an overow pipe 9` through which any excess of waste water may be discharged into the sewer, as shown.

In operation the ventilator gases ow upwardly through the condenser tower 2| and are broken up into a multitude of iine turbulent streams by the packing in the tower to give the eiect of "water from the shower 23 flows downwardly vapor from the rising ventilator gases. The vening tower 32 which forms the second stage of the two-stage tower system 20. The absorbing tower 32 is likewise packed with suitable packing material of an Yinert nature similar to the packing material in the condenser tower 2|. A spray or shower 33 is provided in the top of the absorbing tower 32 which is connected by a pipe -34 to the water supplyl main 25. rIfhe pipe line 34 is provided with a control valve 35. The cool Water from the shower or sprayI 33 passe-s'downwardly through the packed tower 32 and abysorbs the sulphur dioxide from the streamlets of ventilator gases rising upwardly therethrough, to form an aqueous SO2 solution. The aqueous SO2 solution is collected in a shallow collector p an 36 in the base of the absorbing tower 32 as it is formed, and drains into a surge tank or reservoir 31 through a drain pipe 40. The gases leaving the top of the absorption tower 32 will be largely inert, such as nitrogen and carbon dioxide and may be discharged to atmosphere. The

tion will not be formed at a uniform rate. Ac-

cordingly, the waste water storage tank I8 and the reservoir 3`I Vshould have relatively large storage capacities in order to take care of the intermittent ilow of liquids thereto.

The sulphlte plant also includes a sulphur burner gas cooling system indicated generally at `7|! 42. The cooling system 42 shown is of the spray A"I9, The top of the storage tank I8 is vented into large area of contact with the water. The spray` tilator gases leave the top of the condenser tower 2| and pass upwardly into an SO2 absorbtype and includes a first saturating and cooling spray tower 43 and a second larger spray cooling tower 44. The sulphur burner. the acid `tower proper (not shown) and the hot burner gas cooling system 42 operate in a continuous manner as part of the acid liquor makingsystem in contrastq to the intermittently operated digestion system. The acid liquor output of this system is discharged into an acid liquor storage tank (not shown) from which it may be withdrawn as necessary to ll the digester at the start of` each cook or batch. l l

Cooling liquor is continuously circulated through the burner gas cooling system 42. Makeup water must be continuously introduced into the circulating system in order to make up for a loss in this circulating cooling water. The aqueous SO2 solution is utilized in the burner gas cooling system 42 at a uniform rate in connection with this cooling water make-up arrangement as will appear below.

A pump 45 is provided with its inlet or suction side connected to the base of the reservoir 31 through a suitable inlet pipe 46, while the outlet or pressure side of the pipe 45`is connected to a discharge line 4,1.. The discharge requiredy make-up water in addition to the aquei ous SO2 solution supplied by the pump 45 may be added through a pipe line 52 connected with. the water supply main 25. Water from the pipe line 52 discharges into-a compartment 53 and overflows into the compartment 50 over a Weir 54 to form make-up liquor with the aqueous SO2 solution therein. Any excess liquor in. the tank l overflows over another weir 55 into a cornpartment 56 from which it may be withdrawn through a pipe line 51 into the sewer, as shown. A pipe 60 connects the bottom of the compartment 50 with the bottom of the large spray cooling tower 44.

The make-up liquor is withdrawn from the base of the tower 44fby a continuously operating pump 6l. Ther discharge of the pump 6l is divided so that a small part of the liquor flows through a pipe line 62 to the top of the rst cooling tower 43 while the larger part of the cooling liquor passes through a pipe 63 into a heat exchanger 64.

The volume of hot liquor discharged through the line 62 is equal to the aqueous SO2 solution input from the reservoir 31, plus a small circulating load. Thisliquor is sprayed from a shower 65 in the top of the cooling tower 43. Hot SO2 burner gases are introduced into the top of the cooling tower 43 above the shower 45 as indicated at 66, These hot burner gases -flow downwardly through the tower 43 co-current with the finely divided spray liquor from the shower 65 and serve to remove or strip the SO2 content from the spray liquor. The hot burner gases may have a temperature drop from 2000 F. to 160 F. in the tower 43. The waste liquor from which the SO2 has been stripped collects in the bottom of the tower 43 and is drawn off through a line 61 into an outlet Weir tank 10. The overiow compartment of the tank may be connected through a drain pipe 1l to the sewer drain 51, as shown. The partially cooled burner gases leave the bottom of the cooling tower 43 rate of circulation at a phase of the Vprocess wherel the pump Bl is cooled in the heat exchanger 64 by cooling water supplied to the heat exchanger through a pipe line 13 connected to the supply main 2B. Thecooled liquor leaves the top or the heat exchanger through a line 14 through which` it is pumped .to a shower 15 in the top ol the tower 44. The cooling liquor from the shower 15 flows downwardly through the tower 44 countercurrent to the uprising burner gases and serves to. cool .these gases as much as '70 to 80F. The cool SC2, enriched gas leaves the top of the spray cooling tower 44 through the outlet 16 from which it may be conducted to the acid liquor making system (not shown) Substantially all of the SO2 `is stripped from the `spray liquor in the spray cooling ,tower 43 by the hot burner gases flowing downwardly therethrough. This stripped SO2 recovered in the spray cooling tower 43 represents the SO2 dissolved from the blow pit ventilator vgases `in the SO2 absorbing tower 32.

In older burner gas cooling installations wherein the 'coolers are of the surface cooling type, these coolers may be retained to serve` the cooling function` of the cooling tower 44, while the SO2 stripping may be effected in a spray type cooling towensuch as tower 43, placed in series with and ahead of. such surface type coolers. Tli'econdensate from the surface cooler in such an arrangement would be pumped as a circulating load to the stripping tower togetherwith the make-up` in broken line in the drawing, or it may be ashed or conducted into the condenser tower 2| through a line 80, also shown in broken line.

It will be seen that since my invention provides for the recovery and utilization of `sulphur di'- oXide from blow gases, the sulphur dioxide content of the digester contents blown into the blow pit l2 may be substantially higher than. that in the normal practice. This reduces the amount of sulphur dioxide that it is normally necessary to recover during the blow down period of aidigester cook, and thereby shortens the blow-down period. This advantage is of particular importance where modern means for forced circulation are employed, in that the active relief during the latter stages of the cooking cycle greatly impairs the l active and positive circulation is to be desired.

and are conducted into the bottom of the larger spray cooling tower 44 through a` connecting conduit 12.

The larger portion of the liquor discharge from.

Accordingly, each digester may be operated to give increased pulp output for production without a sacrifice in economy due to waste of SO2. Or, as stated above, ifit is not essential or desirable to increase the pulp production, a lower cooking temperature may be used since the eiective cooking period of each batch may be extended a period of time corresponding tov the shortening or reduction of the blow-down period. Y

Since certain'changes may be made in my foregoing method and means for the recovery and., U

utilization of intermittently discharged sulphur dioxide and diiierent embodiments ol `the invention may be made without departing from the spirit and-scope thereof, it is intended that all matter described hereinbefore `or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense, and that the appended claims be given the broadest pos-- from `said storage tank to said burner gas cooling system, and stripping absorbed SO2 from said aqueous SO2 solution by contacting the solution with hot burner gases from said sulphur burner.

2. In the manufacture of sulphite pulp, the method of recovering and utilizing SO2 intermittently discharged as one of the blow pit ventilator gases which comprises, condensing the steam from said blow pit ventilator gases to form hot waste water, maintaining the temperature of said waste water slightly below 212 F. to thereby substantially prevent absorption of SO2 therein, absorbing the SO2 from said blow pit ventilator gases by cool water to form an aqueous solution thereof, conducting said aqueous SO2 solution as it is formed to a' reservoir. therefor, delivering the aqueous SO2 solution at a substantially uniform rate from said reservoir to a. cool-l ing system for cooling sulphur burner gases, and stripping the absorbed SO2 from said aqueous SO2 solution by contacting the solution with hot burner gases from a sulphur burner, said burner gas ycooler system and said sulphurv burner being operated inv a continuous manner in connection with the manufacture of sulphite pulp.

3. In the operation of a sulphite pulp plant which includes at least one digester, a blow pit into which thel dig'ester contents are periodically discharged, -a sulphur burner, and a burner gas cooling system comprising at least a rst saturating spray tower, said sulphur and said burner gas cooler being operated in a continuous manner, the method of recovering and utilizing SO2 intermittently discharged in the blow pit ventilator gas from said blow pit which comprises, conducting blow pit ventilator gas to condenser means, spraying water into said condenser means to condense the water vapor component from said blow pit ventilator gas and form hot waste water,`

thermostaticaiiy regulating the quantity of water sprayed into said condenser so as to maintain said hot waste Water only slightly below 212 F. to thereby substantially prevent absorption of SO2 therein, conducting .the blow pit ventilator gas leaving said condenser means to SO2 absorption means, spraying cool water into said SO2 absorption means to remove the SO2 from said blow pit ventilator gas and to form an aqueous solution thereof, passing said aqueousSO2 solution into a reservoir therefor, delivering the aqueous SO2 solution at a relatively uniform rate from said reservoir to make-up water storage means for said burner gas cooling system to form make-up liquor with makefup water therewithin, pumping said make-up liquor at a rate substantially equal to the rate at which it is delivered from said reservoir plus a small additional circulating load to said first saturating spray tower 8 of said burner gas cooling system, spraying said make-up liquor thus pumped Within first spray tower, and stripping absorbed SO2 from said liquor spray by means of hot burner gases.

4. In the operation of a sulphite pulp plant which includes at least one digester, a blow pit into which the digester contents are periodically discharged, a sulphur burner, and a spray type burner gas cooling system comprising a series of at least first and second spray towers, said sulphur burner and said spray type burner gas cooler being operated in a continuous manner, the method of recovering and utilizing SO2 intermittently discharged in the blow pit ventilator gas from said blow pit which comprises, conducting blow pitl ventilator gas to condenser means, spraying water into said .condensermeans counter-current to the flow of blow pit ventilator gastherein to condense the water vapor therefrom and to form hot wasteV water, thermostatically regulating the rate at which said condenser water is sprayed so as to formv said hot waste water at slightly below 212 F. to thereby substantially prevent absorption of SO2 therein, conducting the blow pit ventilator gas leaving said condenser means into SO2 absorption means, spraying cool water into said SO2 absorption means countercurrent to the flow of `the blow pit ventilator gas therein to remove SO2 therefrom and form an aqueous SO2 solution, collecting said aqueous SO2 solution, passing said aqueous SO2 solution into a reservoir therefor, delivering the aqueous SO2` solution at a relatively uniform rate from said reservoir to make-up water storage means for said burner gas cooling system to form`make-up liquor with make-up water supplied there-to, conducting said make-up liquor tothe last tower in said series of spray towers of said burner gas cooling system, pumping a part of said make-up liquor at a rate substantially equal to the rate at which it is delivered from said reservoir plus a. small additional circulatingload to the top of said rst spray tower of said burner gas cooling system, spraying said make-up liquor thus pumped into said first spray tower, passing hot burner gas into said rst spray tower co-current with said make-up liquor spray to thereby strip the SO2 from this spray, passing the SO2 enriched burner gas into the next in said series of i spray towers, removing the stripped make-up liquor'from said first spray tower, cooling a relatively large part of the liquor from the second cooler by circulating it through a heat exchanger, and spraying the cooled mixture of make-up and circulating liquors into the top of said last spray tower to thereby cool the SO2 enriched burner gas.

HORACE A. DU BOIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNrrED STATES PATENTS Number Number Number Name Date Kahle Nov. 12, 1940 Hofmann Sept. 28, 1915 FOREIGN PATENTS Country Date British Mar. 29, 1923 Norwegian Dec. 9, 1907 British May 8, 1929 

